This section is intended to introduce the reader to various aspects of the art that may be related to various aspects of the present invention. The following discussion is intended to provide information to facilitate a better understanding of the present invention. Accordingly, it should be understood that statements in the following discussion are to be read in this light, and not as admissions of prior art.
Due to the introduction of advanced wireless Radio Access Technologies (RAT), many mobile network operators are forced to deploy systems based on new technology to meet the consumer demands but still need to keep the legacy system for different reasons. Since different RATs may be based on different radio technology, the two RATs deployed by the operator may not be backward compatible. For this reason, inter-RATs mechanisms were developed to allow network to control multi-RATs supported UE switching between different RATs within operator's networks. The inter-RATs mechanism also provide requirements to guide UE to determine which RAT it should be camp on if multiple RATs are detected in the network.
Operators with multiple RATs supported in the network usually configure one of the RAT as the primary RAT. While UEs are in idle, the UEs are usually configured by the operator to search for primary RAT and camp on the primary RAT when possible.
UEs sometime may not camp on the primary RAT due to different reasons. For example, the UE was located in an area only secondary RAT service is provided, and just move into an area with services from both RATs are provided. Another possibility could be that the UE went to secondary RAT to access the network (i.e. with a dedicated channel) because it could not get reasonable channel strength form primary RAT when user is trying to access the network. Regardless how the UE end up camp on the secondary RAT, there are inter-RAT Inter Working (IWK) mechanisms exist to move the UE back to primary RAT when UE is in idle or even when UE has a dedicate channel assigned. Using LTE and eHRPD IWK as an example, two eHRPD (as secondary RAT) mechanisms exists that allows idle UE re-select the LTE (as primary RAT) as the RAT to camp on. The two mechanisms are either based on MMSS (Multi-Mode System Selection), which has minimum control from network, or Other RAT Neighbor List method, which allows network have better control on UE's RAT selection behavior. For UE accessing the eHRPD dedicated channel, the eAN can also use redirection mechanism to handover the UE to LTE system.
The existing inter-RAT mechanisms work well in normal condition, with the assumption that the primary RAT (e.g. LTE) has greater capacity (e.g. allows more UEs attach to the network) compare to the secondary RAT (e.g. legacy system such as HRPD). When overload on primary RAT occurs, the existing infer-RAT mechanisms can also redirect the UE to the secondary RAT to address the overload situation on the primary RAT.
Since inter-RAT functions works both ways. When the UEs are redirected to the secondary RAT due to overload situation on primary RAT, the inter-RAT IWK function on secondary RAT also kick in and attempt to redirect the UE back to the primary RAT since the system is configured primary RAT as the default camp on system for these UE. Since secondary RAT has no knowledge of primary RAT's load condition, UEs may ping-ponging between two RATs and drastically reduce the network efficiency.